The present invention is directed to an open coil electric resistance heater, and in particular to one that employs a twisted electrical resistance wire pair as the heating element thereof.
The use of a single resistance wire formed into a helical coil and then used in an electric heater is well known in the prior art. One example of this application is an open coil electric resistance heater as shown in FIG. 1 and designated by the reference numeral 70.
The heater 70 contains a terminal plate 71, a top cross-beam 72, and three bars 60 that are attached to terminal plate 71. Two heating elements (the top one shown as 74) are disposed on opposite sides of bars 60, with double clinch clips 1 being attached to bars 60 (three clips per bar), and two insulators 20 (only one shown) being attached to each clip 1. The bars 60 are attached at one end to the terminal plate 71 at 60a and on the opposite end to cross beam 72.
The heating elements 74 are each a continuous length of suitable electrical resistance heating wire, such as Nichrome or the like. Preferably, the heating elements are in the form of longitudinal helical coils of the electrical resistance heating wire with the coils each having a multiplicity of generally uniformly spaced convolutions. The heating element 74 has a plurality (e.g., six in FIG. 1) of heating element runs 76, (the coil underneath that is not shown has the same runs). The adjacent runs of the heating elements are electrically connected in series to an adjacent run of the heating element by a looped end turn 78.
In addition to the runs and looped end turns discussed above, the heating element 74 has leads 80, which constitute the ends of the heating elements and which are electrically connected to respective electric terminals 82 in terminal plate 71. Those skilled in the art will recognize that the terminals 82 may be connected to a source of electrical power (not shown) for energizing heating element 74 in the conventional manner.
Heating element 74, via heating element run 76, is supported on insulators 20, thereby holding heating element 74 clear of bars 60 and supporting the heating elements during energization. Each insulator 20 is secured in clip 1 that is supported in turn by the bar 60. This type of heater is disclosed in U.S. Pat. No. 6,509,554 and is herein incorporated in its entirety by reference as one example of an open coil resistance heater.
In these types of heaters, the resistance coils are energized to heat air passing over the coils, the heated air is then useful for different applications, clothes dryers, etc. This is just an example of one type of an open coil resistance heater and many other types exist, such as ones that employ different types of insulator supports and insulators themselves, such as round bushings, point suspension insulators, or flat bushings.
In other heaters, the resistance wires may be coiled in parallel by feeding the wires into a coiling machine for winding on an arbor. Once formed, the individual leads of each separate wire are terminated into a common terminal, one terminal at each end of the coil. This parallel winding is commonly used in sheathed or tubular electric heating elements.
However, there is an ever-pressing need in the open coil resistance heater industry to cut costs of production and use less weight of material. Thus, there is a need for improvements in these types of heating apparatus so that manufacturers can gain competitive edges over their competitors.
In response to this need, the present invention provides a heater that produces a significant savings in weight of material. The inventive open coil resistance heater uses a twisted wire pair in place of the single resistance wire used in conventional open air resistance heating apparatus. Use of the twisted wire pair in the proper configuration results in material weight saving since less wire is ultimately used in making the heating apparatus and the saving is achieved without a loss in heating capacity.
It should be understood that the invention is not the mere use of twisted wires in resistance heaters. The use of twisted wires in electric resistance heating is disclosed in U.S. Pat. No. 5,296,685 to Burstein and U.S. Pat. No. 3,904,851 to Gustafson. Burstein relates to a radiant quartz heater wherein a twisted resistance wire is encased in a quartz tube, and teaches that the pitch or lay distance should be around 9-11 times the diameter of the individual wire. In Gustafson, twisted wires are wound around an insulator plate and a medium is passed over the plate for heating purposes.
Given the fundamental differences between the operating parameters of a quartz radiant heater and open coil resistance heaters, the wire arrangement of Burstein would not work in an open coil resistance heater. As explained by Gustafson, the operating temperature is around 1000 degrees Centigrade, which is much higher than that employed for open coil resistance heaters.
It is a first object of the invention to provide an improved open coil resistance heating apparatus.
It is another object of the invention to provide an open coil resistance heating apparatus that uses less resistance wire weight as compared to conventional open air resistance heating apparatus.
Yet another object of the invention is a method of heating a medium passing over open air coils by using a pair of twisted wires as the resistance wires in replacement for a single wire.
Other objects and advantages will become apparent as a description of the invention proceeds.
In satisfaction of the foregoing objects and advantages of the invention, the invention is an improvement in heaters employing coiled electric resistance wires. According to one aspect of the invention, the single coiled resistance wire used as part of a heater can be replaced with a twisted pair of smaller diameter and specified pitch. The heater can be any type of open air resistance wire heater, and relationship between the single wire and the twisted pair is defined by the equation:
Gauge (single wire)+3.5=gauge (each wire of pair)
For example, a heater using a 16.5 gauge wire can be replaced with a heater using a pair of 20 gauge wires, without loss of performance. Alternatively, a 16 gauge wire heater can be replaced with a heater employing two 19.5 gauge wires.
The pitch of the twisted pair can vary based on a ratio ranging between 25-40, preferably 30-34, wherein the ratio is the pitch divided by the small wire diameter.